Severe Weather Outbreak - July 20th and July 21st, 2007

A deeply amplified polar trough influenced severe weather in the Jacksonville county warning area (CWA) on July 20th and 21st of 2007. While troughs of this amplitude occur periodically, it is rare for this time of year. The associated cold frontal boundary caused weather ranging from a mesoscale convective system (MCS) that produced an expansive gust front to several individual severe thunderstorms that produced hail up to golf ball size, localized flooding, and wind damage. A total of 45 county-based warnings were issued in two days, affecting nearly all of the counties in our CWA.

Friday July 20th, 2007

After several days of above normal temperatures in the Southeast, a deep polar trough (images 1 and 2) began to move into the eastern United States. Temperatures at the surface were still above normal and this in combination with rapid cooling aloft due to the approaching trough created very unstable conditions. This would set the stage for severe weather. A predominately west to southwesterly flow advected deep moisture from the Gulf of Mexico into the area, and was ample for thunderstorm development as shown by precipitable water values near 2 inches (image 3).

Early in the afternoon a mesoscale convective system (MCS) located near Atlanta had begun to intensify and quickly move southeastward. A developing area of differential heating allowed this system to continue to develop and propagate southeastward. As this system continued to develop, the subsequent cold pool maintained the system. By that evening, the gust front itself was moving at a forward speed of around 35 knots, which would add to any additional downbursts that the embedded thunderstorms produced (image 4).

Saturday July 21st, 2007

By Saturday the cold front in association with the deep polar trough was positioned over the Florida / Georgia border (images 5 and 6). Precipitable water was once again near the 2 inch mark, while ample instability due to daytime heating and significant cooling aloft (image 7) led to numerous thunderstorms developing. A jet axis was stretched along the Florida / Georgia border as well, and this in combination with mid level shear and a pocket of mid level dry air would increase the threat of damaging winds and large hail. Additionally, there were several boundaries to provide localized lift, including the front itself, the east coast sea breeze, and any outflow boundaries that would be produced from thunderstorms.

The most significant damage that was reported was in northeast Clay County, Florida from Orange Park south to Green Cove Springs. This cell originated in Charlton County, Georgia, and progressed south to southeastward causing significant damage through Florida, and then finally dissipating over the coastal waters offshore of Flagler County.

See attached loop of composite reflectivity (image 8) from approximately 5 to 6 pm Saturday afternoon to see how the thunderstorm tracked south along an outflow boundary then intensified when it reached an intersection of the Atlantic sea breeze and outflow boundary over Orange Park. The storm then tracked south-southeast across Fleming Island and Green Cove Springs.

The loop of storm relative velocities (image 9) shows a band of very strong straight line winds tracking from Western Duval into northeast Clay County across the Fleming Island area where some radar velocities exceeded 80 knots (92 mph) at times.

On Fleming Island, straight-line winds propagated from the northwest section of the Island over Eagle Harbor and tracked southeast through Fleming Island Plantation exiting along U.S. highway 17 near the Black Creek Overpass. There were numerous reports of trees snapped off and on top of houses as seen in the pictures attached. A house along CR220 and a business along U.S. Highway 17 sustained significant damage to their roofs. Winds based on radar imagery were likely between 70 to 90 mph.

Image 1: Surface weather map at 7:00 am, July 20, 2007

Image 2: 500 mb weather plot at 7:00 am, July 20th, 2007

Image 3: 12Z (8 am LDT) sounding on July 20th, 2007. Hatched area shows large instability due to warm temperatures at the surface, and cooling aloft. Table shows several severe weather parameters.

Image 4: Three hour visible satellite loop of the mesoscale convective system (MCS) and gust front moving through Georgia. Loop is from 2015Z to 2315Z (4:15 pm LDT to 7:15 pm LDT)

Image 5: Surface weather map at 7:00 am on July 21, 2007.

Image 6: 500 mb weather map at 7:00 am, July 21, 2007.

Image 7: 00Z (8 pm LDT) sounding on July 21, 2007. Yellow hatched area shows large amount of instability due to warm surface temperatures and cooling aloft. Blue hatched area shows mid level dry air, which would enhance hail development and local downburst winds. Wind profile to the right shows jet maxima over 60 kt. Table to the right shows several severe weather parameters.